Consider a matrix X ∈ R n × m One compact yet unclear notation to select a row or column from this matrix is: x ∈ XHow do you clearly select a row or column from a matrix?I know X = ( x i j ) is a standard notation to select elements. Though I haven't seen this used, x i ∈ X i j for rows and x j ∈ X i j for columns might make sense. This is motivated by a similar notation I have seen, namely ∑ i X i j for row sum or ∑ j X i j for column sum.

Question

Consider a matrix   X  ∈            R              n      ×      m      One compact yet unclear notation to select a row or column from this matrix is:  x  ∈  XHow do you clearly select a row or column from a matrix?I know   X  =  (      x          i      j        ) is a standard notation to select elements. Though I haven&#039;t seen this used,       x    i    ∈      X          i      j       for rows and       x    j    ∈      X          i      j       for columns might make sense. This is motivated by a similar notation I have seen, namely       ∑          i            X          i      j       for row sum or       ∑          j            X          i      j       for column sum.

dasse9 · Accepted Answer

You could for example use elementary vectors. Let       e    j   be an   m  ×  1 vector of zeros with a one in j-th position. Then      x    j    =  X      e    j  would be the j-th column of X.